There is a great deal of interest in how metal fasteners installed in carbon fiber composite (CFC) aircraft structures respond to high levels of electrical currents conducted through them. Specifically, the concern is about the discharge pattern of arcing and/or sparking occurring as a result of the electrical current passing through the fastener installation. This concern exists because of the ignition hazard which is present when current-caused arcing and/or sparking occurs in a hazardous ignitable vapor area such as fuel tanks in airplanes.
Much laboratory testing has been done in an effort to determine the arcing and/or sparking characteristics of fastener installations in response to conducted electrical currents. The results of these laboratory tests have not been conclusive enough to establish a reasonable confidence in test-determined fastener arc and/or spark current threshold levels.
The primary difficulty in obtaining reliable test data centers around being able to develop a test method which generates data that is not filled with so many unknown variables that it defies analysis. To date, substantial portions of test data have had to be looked upon with suspicion and caution or totally disregarded because of the discovery that some aspect of the test setup or method has influenced, masked, or otherwise called into question the reliability of the test data.
Being able to accurately determine arcing and/or sparking characteristics of fastener installations is important to protecting CFC aircraft from fuel tank ignition caused by electrical current conduction through these fastener installations. Future CFC aircraft must be protected from electrical current-caused arcing and/or sparking resulting from currents introduced to the CFC structure from lightning, electromagnetic pulse (EMP) and nuclear EMP coupling, or electrical system fault currents. Therefore, it can be important to not only recognize the conditions under which arcing and/or sparking occurs, but also to systematically study the locations which are most susceptible to arcing and/or sparking.
As disclosed by Gunn in U.S. Pat. No. 2,586,815, it has long been known to recognize and optically record the breakdown of the electrical insulation in an antenna attached to a radio receiver. This is accomplished by an indicator and recorder which can both cause recognizable audio signals in the receiver and produce an observable and recordable light signal. However, this indicator and recorder are incapable of producing images of the breakdown which can be analyzed in order to determine its likelihood of occurring subsequently.
It is therefore desirable to have a method and an apparatus for systematically producing images of the arcing and/or sparking characteristics of typical fastener installations.